With the vehicle braking systems currently in production there is no way, other than the use of extremely complex external systems, to detect, when in use on a vehicle, the forces that are exchanged in use between the brake pads (or the brake shoes for vehicles still equipped with drum brakes) and the element to be braked, the disk or drum, which is bound to the wheel.
Normally two brake pads per vehicle disc (and therefore per wheel) are controlled by a mechanical caliper which is actuated by a hydraulic and/or electrical circuit when using the vehicle and that exerts pressure on the pads that is proportional to the force applied to the brake pedal. As pressure (nominal) is exerted by the caliper, the pads are pushed against the disc exerting a force that opposes the rotation of the disc, thus braking the vehicle.
The varying pressure distribution that results during the braking process causes unwanted brake pad movements within the caliper itself thus generating different phenomena known in the field of brake pads as, for example, residual torque, vibration and noisiness.
The presence of imperfections within the braking system for example, due in most cases to the wear of the mechanical parts employed (in particular the disc, caliper and brake pads), lead to contact between the surfaces designed to perform the braking which over time is non-homogeneous. Above all, as a result of heavy braking, it may occur that contact remains between the pads and the disc even after the foot is lifted away from the brake pedal, thus resulting in the phenomenon called “residual torque”. Residual torque can also be generated by the formation of lobes on the disc, which may already be present when first mounted or that form as a result of excessive disc overheating.
With respect to noisiness, this can be generated at different frequencies within the range of 0.1 and 20,000 Hz.
It is not currently possible to detect the pressure distribution other than in a static manner using pressure-sensitive paper or else by means of systems that are directly interfaced with computers, such as for example the Tekscan I-Scan® pressure mapping system (http://www.tekscan.com/brake-pad-pressure-distribution).
Neither is it possible to detect the type of noisiness, if not with complex equipment such as microphones and accelerometers positioned as necessary within the braking system; whether the braking system is mounted on a chassis dynamometer or else on the vehicle.
This means that, with respect to issues arising from the non perfect functioning of the brake caliper rather than the pad or disc, current vehicle braking systems are “blind”.
EP1531110 and GB2478423, which describe vehicle braking systems wherein piezoelectric sensors are arranged, respectively, on the brake disc or between the piston and backplate of the brake calipers (the “backplate” is the metallic support element of the friction material of the brake pad that constitutes the carrier element) for the purpose to, respectively, produce a signal that is employed by an electric motor to adjust the position of the brake caliper piston or else to detect any signs of wheel locking during braking, do not solve this problem.
EP1431606B1 describes a method for the measurement of forces applied to a layer of friction material wherein a functional layer, whose electrical resistance varies as a function of the forces applied to it, is associated with said layer of friction material; the variation in the electrical resistance of the functional layer is then measured and is in proportion to the magnitude of the applied forces.
EP1923592B1 instead describes a brake or friction element having a friction layer and a support plate with at least one capacitive sensor arranged between the friction layer and the support plate, the capacitance of which varies as a function of the force applied to said friction layer.
US2006/0254868 describes a system similar to that of EP1431606B1, wherein the variation in the electrical resistance of a brake element layer of friction material, such as a brake pad, is measured directly.
Even the systems described in EP 1431606B1, EP1923592B1 and US2006/0254868 do not solve the above mentioned technical problem, since they are based on the electrical capacity or electrical resistance variation of a sensor or of an entire functional layer, that allow with relative precision for only static, or very slow, detection of the forces applied to the brake pad, but that are not capable of detecting rapid force variations such as those that occur during braking.
Moreover, these systems need to be continuously supplied electrically, which entails drawbacks, such as the relative amount of energy consumed and the considerable constructive complications associated with ensuring the supply of electrical power between parts that are in relative rapid motion.